Gas generators are used in a wide variety of applications, one of which is in automotive safety systems. For instance, gas generators are used to provide propellant gases for expanding air/safety bags to protect motor vehicle occupants from an impact of a predetermined magnitude. Gas generators are also used in motor vehicle seat belt restraint systems, particularly in a pretensioner which may be provided for each particular seat belt assembly. Generally, a pretensioner removes slack existing in the associated seat belt assembly upon the sensing of an appropriate condition, such as a predetermined magnitude of deceleration. For example, the propellant gases provided by the gas generator may be used to axially advance a portion of the pretensioner interconnected with the associated seat belt assembly to remove existing slack. Moreover, the propellant gases provided by the gas generator may rotate the associated seat belt retracting device to remove the existing slack.
Gas generators may be electrically or mechanically activated. In electrically-activated gas generators, generally an appropriate sensor sends an electrical signal to the gas generator to initiate combustion of the propellant contained therein. The sensor is typically remotely located such that the signal is sent along one or more lead wires which are interconnected with a squib or other detonating device associated within the gas generator's ignition assembly. In mechanically-activated gas generators, one or more primers (e.g., conventional stab or percussion primers) ignite the propellant after a firing pin engages the primer(s) to ignite the primer composition which then, directly or indirectly, ignites the propellant.
Mechanically-activated gas generators in automotive safety systems provide a number of advantages over those which are electrically initiated. For instance, the sensor(s) of electrically-activated gas generators are typically remotely positioned relative to the gas generator and thus require long lead wires along which the signal(s) must travel. In addition, relatively complex electronic circuitry is typically required, such as to ensure that a signal is sent only for appropriate conditions. Consequently, high material costs and/or reliability issues are often associated with electrically-initiated gas generators.
Mechanically-activated gas generators often incorporate the sensor proximate the gas generator and make such an integral part of the ignition assembly, thereby avoiding problems associated with long lead wires interconnected to squibs in electronic systems. Generally, principles of inertia are utilized to initiate the advancement of a firing pin into engagement with the associated primer, such as based upon a certain deceleration. Notwithstanding the benefits associated with mechanically-activated gas generators, characteristics associated with conventional primer technology presents certain difficulties. For instance, conventional percussion primers typically require significantly high impact energies in order to achieve ignition of conventional primer compositions. As a result, the associated mechanical crash sensor must utilize relatively significant masses to ensure that the required energy transfer occurs. Although conventional stab primers may have reduced impact energy requirements, the more sensitive compositions associated with these primers have actually impeded their use in automotive applications. More particularly, in order to simulate aging, much of the automotive industry requires that all propellants must be able to withstand exposure to a temperature of 107.degree. C. for 400 hours and yet still properly ignite thereafter. Some stab primer compositions are unable to meet this standard.
Based upon the foregoing, it would be desirable to take advantage of the benefits offered by mechanically-activated gas generators. However, it would be desirable to have reduced primer energy input requirements and yet still meet current automotive industry standards.